add translation keys (#246)

* Fix i18n tsc issue

"t" can return null by default.
This was already disabled in the config, but it had no effect on tsc.

* Fix translation in "DefaultNavBar"

key was missing and since this is a constant the translation wouldn't update in case the language got changed after first load.

* Enable typing for i18n translation keys

* Add translation keys

src/i18n/resources/ar.json

@@ -1,7 +1,5 @@
 {
-    "screens": {
-        "Library": {
-            "Library": "المكتبة"
-        }
+    "library": {
+        "title": "المكتبة"
     }
 }